Problem: Lines $l$ and $m$ are parallel to each other. $m\angle A = 120^\circ$, and $m\angle B = 150^\circ$. What is the number of degrees in $m\angle C$?

[asy]
size(100); real h = 1.2; currentpen = fontsize(10pt);
draw(Label("$l$",Relative(1)),(0,0)--(1,0),E);
draw(Label("$m$",Relative(1)),(0,-h)--(1,-h),E);
draw((0,-h)--h/2*(cos(150*pi/180),sin(150*pi/180)) + (0,-h));
draw(Label("$C$",Relative(1)),(0,0)--h*sqrt(3)/2*(cos(-120*pi/180),sin(-120*pi/180)),W);
label("$A$",(0,0),N); label("$B$",(0,-h),S);
label("$120^\circ$",(0,0),SE); label("$150^\circ$",(0,-h),NE);
[/asy]
We draw segment $\overline{CD}$ as shown parallel to lines $l$ and $m$.

[asy]
size(200); real h = 1.2; currentpen = fontsize(10pt);
draw(Label("$l$",Relative(1)),(0,0)--(1,0),E);
draw(Label("$m$",Relative(1)),(0,-h)--(1,-h),E);
draw((0,-h)--h/2*(cos(150*pi/180),sin(150*pi/180)) + (0,-h));
draw(Label("$C$",Relative(1)),(0,0)--h*sqrt(3)/2*(cos(-120*pi/180),sin(-120*pi/180)),W);
label("$A$",(0,0),N); label("$B$",(0,-h),S);
label("$120^\circ$",(0,0),SE); label("$150^\circ$",(0,-h),NE);
pair D = (h*sqrt(3)/2*(cos(-120*pi/180),sin(-120*pi/180))) + (2,0);
draw(D -- (D-(2,0)));
dot(D);
label("$D$",D,E);
[/asy]

Since $\overline{CD}\parallel l$, we have $\angle ACD = 180^\circ - \angle A = 60^\circ$.  Since $\overline{CD}\parallel m$, we have $\angle BCD = 180^\circ - \angle B = 30^\circ$.  Therefore, $\angle ACB = \angle ACD + \angle DCB = 60^\circ + 30^\circ = \boxed{90^\circ}$.